This invention relates to a method and an apparatus for controlling the contrast range of the recording light falling on an image recording medium.
When taking pictures with film-based or electronic still cameras or when recording image sequences by means of a motion picture or video camera, great differences in brightness can occur in the object, i.e. a great dynamic range in the object to be recorded, for example in the case of regions with intensive sunlight and very dark shades. This dynamic range can be much greater than the technically realizable dynamic range of an image recording medium, for example of a light-sensitive film material or of electronic image sensors in the form of CCD or C-MOS video sensors. The various image recording media have dynamic or contrast ranges of different sizes admissible for the recorded image and greatly differ in their behavior when exceeding the admissible range.
While a color negative film has a very large dynamic range and hence a very large contrast range and its characteristic slowly levels off, so that even when exceeding the admissible range and hence in the case of over- and underexposures, corresponding image information can still be stored on the color negative film, so that this film material generally is unproblematic in the case of very great differences in brightness of an object, electronic image sensors have a very much smaller dynamic range. What complicates things with electronic image sensors is the fact that their linear sensitivity characteristic does not level off slowly, but abruptly bends down upon reaching the maximum saturation. An electronic processing of the converted image signals emitted by electronic image sensors by means of signal amplification or gamma correction neither leads to a significant expansion of the dynamic range of the electronic image sensors, but only to a negative influence on other parameters, for example to an amplification of noise signals.
Another method known under the term “Correlated Double Sampling” is very complex in electronic terms, and can be performed only with moderate success in particular when recording moving images, because of the required double recording and the changing locations of the objects due to the movement.
Another possibility to take into account the limited dynamic and contrast ranges in particular of electronic image sensors consists in lightening dark regions of the object to be recorded by means of mirrors or lampheads, which involves, however, a considerable amount of time and effort.
From U.S. Pat. No. 4,900,131 a camera attachment with a means for reducing the contrast range is known, which consists of a glass pane with a plurality of light sources distributed around the periphery of the glass pane, which couple light into the plane of the glass pane, which light is reflected inside the glass pane and thereby provides for controlling the contrast range of the object to be recorded without impairing the resolution of the recorded image. For uniform illumination of the glass pane and hence of the image to be recorded, the light sources are arranged in the vicinity of the periphery of the glass pane, whose peripheral surface is polished and mirrored with the exception of the portions opposite the light sources, in order to prevent an exit of light via the peripheral surface, whereas the planar surfaces of the glass pane are provided with an anti-reflection coating, in order to achieve a maximum transparency for the image recording optical path from the object to be recorded.
A disadvantage of the known apparatus for contrast reduction is the coupling in of a light mixture whose spectral distribution or color temperature depends on the type of light source used and its temperature, so that it is possible to selectively influence the light output of the light source by feeding the light source with a variable voltage without changing the color temperature, but the efficiency of the light coupling for contrast reduction of the recorded image furthermore is limited in that individual colors of the light mixture can go beyond the admissible dynamic range and hence determine the total dynamic range of the recorded image, while the contrast range of other colors still lies within the admissible dynamic range.